Apparatus for driving an electro-luminescent device

ABSTRACT

An apparatus ( 10 ) for driving an electro-luminescent device ( 12 ), characterized by: a transformer ( 14 ) including a ferrite core ( 72 ) and a first primary winding ( 66 ), a second primary winding ( 68 ) and a secondary winding ( 70 ) wound onto said core ( 72 ) without an air gap; one end of each of the first and second primary windings ( 66, 68 ) being connected together; an electrical circuit arranged to drive said one ends of the first and second primary windings ( 66, 68 ); a feed back circuit connected between the electrical circuit and the other end of the second primary winding ( 68 ) to control operation of said electrical circuit; one end of the secondary winding ( 70 ) being connected to said other end of the second primary winding ( 68 ); and the other end of the secondary winding ( 70 ) being connected to the electro-luminescent device ( 12 ).

FIELD OF THE INVENTION

The present invention relates to an apparatus for driving anelectro-luminescent device.

DISCUSSION OF THE PRIOR ART

Electro-luminescent sheets are used in a variety of applications,including signs and house numbers. Electro-luminescent sheets typicallycomprise a conductive substance sandwiched between two conductiveplates. The conductive substance becomes luminescent in the presence ofan electric field, so that by applying a voltage across the twoconductive plates, the conductive substance emits light.

Electro-luminescent sheets currently available require an AC voltage tobe applied to the conductive plates in order to achieve luminescence.Inefficiencies in existing DC-AC inverter circuits have limited the useof electro-luminescent sheets where only battery power is available.

Typically, an electro-luminescent sheet requires 40-80 volts, dependingupon the brightness required. Existing DC-AC inverter designs consumeapproximately 80 milliamps or more to achieve the required AC voltage.

Throughout this specification, unless the context requires otherwise,the word “comprise” or variations thereof, will be understood to implythe inclusion of a stated integer or group of integers but not theexclusion of any other integer or group of integers.

BRIEF DESCRIPTION OF THE INVENTION

According to one aspect of this invention, there is provided anapparatus for driving an electro-luminescent device, characterized by: atransformer including a ferrite core and a first primary winding, asecond primary winding and a secondary winding wound onto said corewithout an air gap; one end of each of the first and second primarywindings being connected together; an electrical circuit arranged todrive said one ends of the first and second primary windings; a feedback circuit connected between the electrical circuit and the other endof the second primary winding to control operation of said electricalcircuit; one end of the secondary winding being connected to said otherend of the second primary winding; and the other end of the secondarywinding being connected to the electro-luminescent device.

Preferably, the electrical circuit includes a transistor,

Preferably, the feed Pack circuit comprises a resistor and a capacitorprovided in series between said other end of the secondary primarywinding and the base of the transistor.

Preferably, the resistor and the capacitor are arranged such that thetransistor produces an oscillating output having a frequency less than200 Hz.

Preferably, the apparatus includes a timing circuit arranged todeactivate said electrical circuit after a predetermined time period.

Preferably, the apparatus further includes a light sensing circuitarranged such that said electrical circuit is deactivated in thepresence of daylight or other strong light sources.

Preferably, the turns ratio between the first and second primarywindings is between 1:14 and 1:12. More preferably, the turns ratiobetween the first and second primary windings is 1:13⅓.

Preferably, the second primary winding has 16.5 turns.

Preferably, the turns ratio between the second primary winding andsecondary winding is between 1:100 and 1:250. More preferably; the turnsratio between the second primary winding and the secondary winding isbetween 1:175 and 1:185. Still more preferably, the turns ratio betweenthe second primary winding and the secondary winding is between 1:181and 1:182.

Preferably, the secondary primary winding is wound onto said core first,then said second primary winding and finally said first primary winding.

According to a second aspect of the present invention, there is providedan electro-luminescent device, characterized in that said devicecomprises at least in part a driver apparatus, said driver apparatus inturn comprising: a transformer including a ferrite core and a firstprimary winding, a second primary winding and a secondary winding woundonto said core without an air gap; one end of each of the first andsecond primary windings being connected together; an electrical circuitarranged to drive said one ends of the first and second primarywindings; a feed back circuit connected between the electrical circuitand the other end of the second primary winding to control operation ofsaid electrical circuit, one end of the secondary winding beingconnected to said other end of the second primary winding; and the otherend of the secondary winding being connected to the electro-luminescentdevice.

According to a further aspect of this invention, there is provided anapparatus for driving an electro-luminescent device, characterised by: atransformer, an electrical circuit arranged to drive the transformer; afeed back circuit connected between the electrical circuit and thetransformer to control operation of said electrical circuit; a lightsensing circuit; and a timing circuit arranged to deactivate saidelectrical circuit after a predetermined time period.

Preferably, the light sensing circuit comprises at least in part a lightdependent resistor and a resistor arranged to provide bias voltage to aNAND gate. The light dependent resistor and the resistor bias bothinputs of the NAND gate where NAND gate acts as an inverter. Stillpreferably, the light dependent resistor and the resistor are arrangedsuch that in the presence of strong light the output of the NAND gatewill be low and in darkness the output of the NAND gate will be high.

The output of the NAND gate charges or discharges a capacitor via aresistor. A diode is connected between the capacitor and the positivevoltage rail to clamp the voltage across the capacitor. The capacitor,the resistor and the diode form a damping circuit for the output of theNAND gate.

A NAND gate is connected to the capacitor. The output of the NAND gateacts as an input to a NAND gate. A capacitor and a resistor areconnected to the output of the NAND gate to form a high-pass filter. Thevoltage across the resistor is output to the timing circuit.

The capacitor and the resistor are chosen such that the output of theNAND gate must be high for 4 minutes before the voltage across thecapacitor is sufficient to trigger the NAND gate. This is desirable toprevent temporarily bright lights, such as those from passing vehicles,re-triggering the timing circuit. The capacitor and the resistor act toprovide a pulse signal to the timing circuit.

The timing circuit comprises an integrated timing circuit, a crystal, adiode, a resistor and a capacitor. The integrated timing circuit isconnected to the resistor and receives the pulse signal therefrom. Thecrystal is connected to the integrated timing circuit to provide astable frequency source. The integrated timing circuit is arranged toprovide a pulse output for 7 hours after being triggered. The diode, theresistor and the capacitor act as a smoothing circuit such that whilstthe integrated timing circuit is providing a pulse output, the voltageacross the capacitor is high.

The control circuit comprises a transistor biased by resistors andconnected between the position voltage rail and the output of a NANDgate such that the transistor will be switched on only when the outputof the NAND gate is low. The collector of the resistor is connected tothe base of the transistor via a resistor.

One of the inputs of the NAND gate is connected to the output of theNAND gate and the other input of the NAND gate is connected to thecapacitor. Since both inputs must be high in order that the output ofthe NAND gate be low, and consequently the transistor be switched on,power will not be supplied to the electro-luminescent sheet unless thetiming circuit is active and light is not sensed by the light dependantresistor

DESCRIPTION OF THE DRAWINGS

One embodiment of this invention will now be described with reference tothe accompanying drawing, in which:

FIG. 1 is a schematic circuit diagram of an apparatus according to thefirst embodiment.

DESCRIPTION

The first embodiment is directed towards an apparatus 10 for driving anelectro-luminescent sheet 12.

The apparatus 10 comprises a transformer 14, a transistor 16, a feedback circuit 18, a control circuit 20, a timing circuit 22 and a lightsensing circuit 24. The apparatus also includes a power supply in theform of a battery 26 and a capacitor 28.

The light-sensing circuit 24 includes a light dependent resistor 26 anda resistor 28 arranged to provide bias voltage to a NAND gate 30. Thelight dependent resistor 26 and the resistor 28 bias both inputs of theNAND gate 30, such that the NAND gate 30 acts as an inverter The lightdependent resistor 26 and the resistor 28 are arranged such that in thepresence of strong light, such as sunlight, the output of the NAND gate30 will be low and in darkness the output of the NAND gate 30 will behigh.

The output of the NAND gate 30 charges or discharges a capacitor 32 viaa resistor 34. A diode 36 is connected between the capacitor 32 and thepositive voltage rail to clamp the voltage across the capacitor 32. Thecapacitor 32, the resistor 34 and the diode 36 form a damping circuitfor the output of the NAND gate 30.

A NAND gate 38 is connected to the capacitor 32. The output of the NANDgate 38 acts as an input to a NAND gate 40. A capacitor 42 and aresistor 44 are connected to the output of the NAND gate 40 to form ahigh-pass filter. The voltage across the resistor 44 is output to thetiming circuit 22.

The capacitor 32 and the resistor 34 are chosen such that the output ofthe NAND gate 30 must be high for 4 minutes before the voltage acrossthe capacitor 32 is sufficient to trigger the NAND gate 38. This isdesirable to prevent temporarily bright lights, such as those frompassing vehicles, re-triggering the timing circuit 22. The capacitor 42and the resistor 44 act to provide a pulse signal to the timing circuit22.

The timing circuit 22 comprises an integrated timing circuit 46, acrystal 48, a diode 50, a resistor 52 and a capacitor 54. The integratedtiming circuit 46 is connected to the resistor 44 and receives the pulsesignal therefrom. The crystal 48 is connected to the integrated timingcircuit 46 to provide a stable frequency source. The integrated timingcircuit 46 is arranged to provide a pulse output for about 7 hours afterbeing triggered. The diode 50, the resistor 52 and the capacitor 54 actas a smoothing circuit such that whilst the integrated timing circuit 46is providing a pulse output, the voltage across the capacitor 54 ishigh.

The control circuit 20 comprises a transistor 56 biased by resistors 58and 60 connected between the position voltage rail and the output of aNAND gate 62 such that the transistor 56 will be switched on only whenthe output of the NAND gate 62 is low. The collector of the resistor 56is connected to the base of the transistor 16 via a resistor 64.

One of the inputs of the NAND gate 62 is connected to the output of theNAND gate 30 and the other input of the NAND gate 62 is connected to thecapacitor 54. Since both inputs must be high in order that the output ofthe NAND gate 62 be low, and consequently the transistor 56 be switchedon, power will not be supplied to the electro-luminescent sheet 12unless the timing circuit 22 is active and light is not sensed by thelight dependant resistor 26.

The transformer 14 comprises a first primary winding 66, a secondprimary winding 68 and a secondary winding 70 wound onto a ferrite core72 without an air gap. The secondary winding 70 consists of 3000 turnswhich are wound onto the ferrite core 72 first. Next, the second primarywinding 68 which comprises 16.5 turns is wound onto the ferrite core 72.Finally, the first primary winding 66 which comprises 220 turns is woundonto the ferrite core 72. One end of the first and second primarywinding 66 and 68 are connected together and to the emitter of thetransistor 16. The other and of the first primary winding 66 isconnected to ground. The other end of the second primary winding 68 isconnected to one end of the secondary winding 70. The other end of thesecondary winding 70 is connected to the electro-luminescent sheet 12.

The feed back circuit 18 extends between the other end of the secondprimary winding 68 in the base of the transistor 16. The feed backcircuit 18 comprises a resistor 74 and a capacitor 76 provided inelectrical series, The feed back circuit 18 ensures the transistor 16oscillates on and off, with the values of the resistor 74 and thecapacitor 76 chosen such that the frequency of oscillation is 152 Hzwith a battery 26 of 3 volts.

The circuit shown in FIG. 1 produces a voltage of 37-40 AC across theelectro-luminescent sheet 12, which is sufficient to ruminateapproximately 100 square centimeters of the sheet 12.

It should be appreciated that this invention is not limited to theparticular embodiment described above.

For example, the apparatus could be powered by sources other thanbatteries, such as solar panels or mains power. Where mains power isavailable, the timing circuit may be omitted. Further, a combination ofpower sources, such as a solar panel with a battery, or mains power witha battery back up could be utilized.

What is claimed is:
 1. An apparatus for driving an electro-luminescentdevice, characterized by: a transformer including a ferrite core and afirst primary winding, a second primary winding and a secondary windingwound onto said core without an air gap; one end of each of the firstand second primary windings being connected together; an electricalcircuit arranged to drive said one ends of the first and second primarywindings; a feed back circuit connected between the electrical circuitand the other end of the second primary winding to control operation ofsaid electrical circuit; one end of the secondary winding beingconnected to said other end of the second primary winding; and the otherend of the secondary winding being connected to the electro-luminescentdevice, wherein the secondary primary winding is wound onto said corefirst, then said second primary winding and finally said first primarywinding, the turns ratio between the first and second primary windingsbeing from 1:14 to 1:12, the turns ratio between the second primarywinding and secondary winding being from 1:100 to 1:250; and whereinsaid electrical circuit includes a transistor and said feed back circuitcomprises a resistor and a capacitor provided in series between saidother end of the second primary winding and the base of the transistor.2. An apparatus according to claim 1, wherein the resistor and thecapacitor are arranged such that transistor produces an oscillatingoutput having a frequency less than 200 Hz.
 3. An apparatus according toclaim 1, wherein the apparatus includes a timing circuit arranged todeactivate said electrical circuit after a predetermined time period. 4.An apparatus according to claim 1, wherein the apparatus furtherincludes a light sensing circuit arranged such that said electricalcircuit is deactivated in the presence of daylight or other strong lightsources.
 5. An apparatus for driving an electro-luminescent device,characterized by: a transformer including a ferrite core and a firstprimary winding, a second primary winding and a secondary winding woundonto said core without an air gap; one end of each of the first andsecond primary windings being connected together; an electrical circuitarranged to drive said one ends of the first and second primarywindings; a feed back circuit connected between the electrical circuitand the other end of the second primary winding to control operation ofsaid electrical circuit; one end of the secondary winding beingconnected to said other end of the second primary winding; the other endof the secondary winding being connected to the electro-luminescentdevice; and a timing circuit arranged to deactivate said electricalcircuit after a predetermined time period, wherein the secondary primarywinding is wound onto said core first, then said second primary windingand finally said first primary winding, the turns ratio between thefirst and second primary windings being from 1:14 to 1:12, and the turnsratio between the second primary winding and secondary winding beingfrom 1:100 to 1:250.
 6. An apparatus according to claim 5, wherein thelight sensing circuit comprises at least in part a light dependentresistor and a resistor arranged to provide bias voltage to a NAND gate,the light dependent resistor and the resistor biassing both inputs ofthe NAND gate such that the NAND gate acts as an inverter.
 7. Anapparatus according to claim 6, characterized in that the lightdependent resistor and the resistor are arranged such that in thepresence of strong light the output of the NAND gate will be low and indarkness the output of the NAND gate will be high.
 8. An apparatusaccording to claim 6, characterized in that the output of the NAND gatecharges or discharges a capacitor via a resistor and a diode isconnected between the capacitor and the positive voltage rail to clampthe voltage across the capacitor, whereby the capacitor, the resistorand the diode form a damping circuit for the output of the NAND gate. 9.An apparatus according to claim 8, characterized in that a second NANDgate is connected to the capacitor whereby the output of the NAND gateacts as an input to a third NAND gate, a capacitor and a resistor beingconnected to the output of the third NAND gate to form a high-passfilter.
 10. An apparatus according to claim 9, characterized in that thecapacitor and the resistor are chosen such that the output of the NANDgate must be high for 4 minutes before the voltage across the capacitoris sufficient to trigger the second NAND gate such that bright lights,such as those from passing vehicles, do not re-trigger the timingcircuit.
 11. An apparatus according to claim 5, characterized in thatthe timing circuit comprises an integrated timing circuit, a crystal, adiode, a resistor and a capacitor, the integrated timing circuit beingconnected to the resistor and receives the pulse signal therefrom andthe crystal being connected to the integrated timing circuit to providea stable frequency source.
 12. An apparatus according to claim 11,characterized in that the integrated timing circuit is arranged toprovide a pulse output for about 7 hours after being triggered.